The vast majority of the world's copper resources are in the form of low-grade (e.g., assaying less than about 1% copper) porphyry copper deposits. In porphyry copper deposits, copper occurs primarily as a copper sulfide and in particular chalcopyrite (CuFeS.sub.2) or chalcocite (Cu.sub.2 S). A sulfide is a compound in which a metal, such as copper, is bonded to one or more sulfur atoms. Other copper sulfides include bornite and covellite.
As a result of post-deposition oxidation of chalcopyrite, deposits typically are zoned with the shallower portions containing copper oxides underlain by chalcocite and the deeper portion containing chalcopyrite with little or no copper oxides or chalcocite. An oxide is a compound in which a metal, such as copper, is bonded to one or more oxygen atoms. Copper oxides include chrysocolla, malachite, cuprite and azurite.
Porphyry copper deposits typically contain other types of minerals associated with the copper minerals. For example, porphyry copper deposits typically contain a significant but highly variable amount of other sulfides associated with copper sulfides, particularly pyrite.
Post-deposition oxidation of the deposit causes the ore to have different proportions of chalcocite, chalcopyrite, other sulfides and copper oxides in the shallower portions of the deposit. The high degree of variability in the proportions of the copper sulfides and copper oxides throughout the deposit renders it difficult and costly to selectively mine copper oxides from copper sulfides such as chalcocite and chalcopyrite.
Different techniques are employed to recover copper from chalcocite and chalcopyrite on the one hand and copper oxides on the other. For copper sulfides, flotation processes are widely used to separate copper sulfides from copper oxides and the remaining ore materials, with the copper in the copper sulfides being recovered by smelting. In contrast, ores predominantly containing copper in the form of copper oxides are typically leached by heap leaching techniques to solubilize the copper oxides.
Flotation processes generally separate the copper sulfides from other sulfides and copper oxides by collecting the copper sulfides in the flotation froth. The froth is removed as a concentrate to be treated by a smelter and the residue is removed as tailings for discard. To form the concentrate, air bubbles are passed through a slurry containing ore particles to form the froth containing air bubbles attached to particles having exposed copper sulfide minerals. The necessary hydrophobic properties of the copper sulfide minerals can be established and recovery of copper sulfides obtained by contacting the slurry with a collector. Typically, the collector contains a hydrocarbon radical attached to a polar group. The polar group attaches to the copper sulfide mineral surface and the hydrocarbon radical which attaches to an air bubble. The particles containing exposed copper sulfide minerals attached to a collector are carried upward by the air bubbles to the froth. The particles in the tailings remain in the flotation cell contents for discharge.
The flotation process generally involves a significant loss of copper to the tailings because of the need to produce a concentrate for treatment by a smelter. Smelters generally require concentrates to assay at least 25% copper to obtain acceptable technical and economic results. To produce such a concentrate, the flotation process is usually designed to include in the concentrate only particles containing higher proportions of copper. To realize such a result, selective collectors and specific flotation conditions are used to recover higher grade particles while suppressing the recovery of lower-grade particles that would reduce the concentrate assay to less than 25% copper. Selective collectors preferentially attach to copper sulfide minerals and not to other sulfides. While residence times in the flotation circuit can be reduced to suppress the recovery of lower-grade particles, this can also cause the loss of slower floating, higher-grade particles. Smelter grade requirements higher than 25% would require even more lower grade particles to be discharged in the tailings. In flotation processes, copper losses also result from the inability of the collector to attach to copper oxide minerals.
Copper loss by flotation processes is significant. In 1991, for example, copper flotation plants in the U.S. recovered about 1.19 million metric tons of copper in concentrate fractions with a copper recovery of about 83%, and an estimated loss in the tailings fraction of about 228,100 metric tons of copper. Chilean government-owned copper mining operations recovered approximately 1.055 million metric tons of copper, with approximately 247,200 metric tons estimated to be lost in the tailings fraction. Additional losses of potentially valuable materials are incurred by the discard of pyrite and other sulfides.
There is a need to provide a methodology for selecting a process for recovering copper from copper-containing material that is appropriate for the specific composition of the copper-containing material, especially for materials containing variable amounts of copper sulfides and oxides and other minerals.
There is a need to provide a flotation process that provides increased recovery of copper from copper-containing materials. There is a related need to provide such a process that can be incorporated into existing flotation operations.
There is a need to provide a process that recovers an increased amount of copper in the concentrate fraction. There is a related need to provide a process that recovers the copper in both copper sulfide and copper oxide minerals.
There is a need to provide a process that can economically recover copper from a lower grade material (e.g., ore or hydrometallurgical tailing), which cannot be upgraded to smelter requirements.
There is a need to provide a process that recovers copper from a concentrate fraction by techniques other than smelting.